in addition to call manager you'll need a router to act as your h.323 voice gateway as well as a trunk to your pbx to maintain 4 digit dialing during the migration. This will allow you to do it in paralell, step by step. the trunk to the pbx will alllow incoming calls to the ip phones through the pbx and from the ip phones out through the pbx. you could accept a couple of pri's on your gateway for the ip phones to go out and use that same router as your trunk. what you should use is highly dependant on the size of your environment. I would probably use a different router as the trunk but ...
What kind of pbx do you have?
For a larger environment you will probably want 2 call managers and will want to take advantage of srst so many phones can still be used if your Call Manager(s) are down for any period of time or if the link between your phones and CCM(s) are down.
to do it in paralell, you can either configure the pbx to send the 4 digit range of the ip phones to the trunk you create between the 2 so only the ip phone rings or you could set it up so the ip phones have the same numbers as the users regular phone and have them ring similtaniously. either in the same location or in diffferent locations.
many things to consider. Voice mail also needs to be addressed in most cases and for this you could use unity which will also allow for the parelell install. Your unity needs will depend on the type of pbx, whether you'll need to use unity for voicemail for the regular phones during the migration etc.......
